Yup, been there, did that. I had a lot of initial depression when I returned to work from my second dd (a lot earlier than I would have wished), and it seemed to go on longer than with my first. I remember vividly, about a month or two after returning to work, I was feeling stressed, but pretty much like I had it together, and then someone said to me, "Are you okay? You always look so sad these days." That was it...I burst into tears all over the place.
What you are doing is hard--it's very hard. You say you are pumping, and I know for me, that was no picnic either. I was glad to do it, and wouldn't have voluntarily given it up, but a lot of days it was one more thing to schedule in and plan around and be healthy and well-fed and well-rested for, and all that.
I think it will get better, but it may never be easy--does that make any sense? Monday's are still really hard drop-off days for us...last week, my older daughter sat at the breakfast table and asked "Mommy, when will it be another three stay home days?" It seems that two are no longer sufficient for her!
I felt like telling her "Not soon enough, sweetheart!"
As Nissa says, try to remember what it is you are working for, and focus on that. And be open to any change that might make the situation better for you, whether it be part time, better hours, whatever.